Electron beams in electron tubes are formed using either thermionic or field emitters as an electron gun with a control grid or anode to form the electrons to the desired shape. The electron gun and anode produce a specified number of electrons at a specified velocity for use in electron tubes such as beam power tubes. The number of electrons in the electron beam must be constant and controllable, and the energy of all the electrons must be substantially the same for efficient operation. Even small changes in emitter temperature result in changes in the electron emission. Similarly, small changes of anode voltage can affect the current. Therefore, anode potential and emitter temperature must be well regulated to provide constant current for proper operation of a thermionically controlled electron gun or electron beam forming device. A field effect electron gun and control electrode can control electron flow without thermionic emission or thermionic interference. U.S. Pat. No. 3,783,325 issued Jan. 1, 1974 to Joe Shelton discloses a field effect electron gun wherein the number of electrons emitted is a function of the electric field. The electric field which is developed between the emitter and the anode is controlled by the emitter and anode structure.
The usual approach to forming electron beams, using either thermionic or field emitters, is to use a grid to form the electrons to the desired shape. When multiple electron beams are required as for multiple beam cathode ray tubes, several individual emitting sources and control electrodes are required. This approach leads to problems due to electrons being intercepted by the grid resulting in grid heating and excessive grid current.
The maximum current density obtainable from an array or field emitting points has been limited due to edge effects. This is due to the outer emitting points seeing a larger collection area than the inner points which results in a higher electric field at the outer points. This difference in electric field causes the outer points to emit a sufficient number of electrons to damage the outer pins before the electric field reaches a sufficient value at the inner points to initiate emission. This results in a low total current and damage to the outer emitting points.